WCF is great if you want to make a single data service available as JSON via REST, XML via SOAP, a custom binary protocol via raw TCP, and who-knows-what else.
In practice you're almost always better off just picking a single format for your endpoint, in which case there's no reason to use WCF.
In practice you're almost always better off just picking a single format for your endpoint, in which case there's no reason to use WCF.